That happens quite often, you run a registered application but you need to install it to your new computer but you have lost the email you received that contained the serial. Actually there is an easy way to read that serial, just launch the application and select the 'Application name > About the application name' menu, for example for MaxBulk Mailer that menu is 'MaxBulk Mailer > About MaxBulk Mailer':
You will get a window with information about the software version, copyright information and the registration status at the bottom:
Just click on it, the serial number will then be displayed:
Now you just need to copy the serial and enter it again where you need it.
Note that you have to be the owner of the serial, the use of a faked or pirated serial number is illegal and will be prosecuted to the fullest extent of the law. Software piracy is illegal and can have very costly consequences to you and your business.